A Group III nitride semiconductor possess a band gap of a direction transition type of energy corresponding to the visible light to the ultraviolet light region, and has an excellent level of light emission efficiency. Therefore, it has been commercialized as semiconductor light-emitting devices such as light-emitting diodes (LED) and laser diodes (LD) and is used in a variety of purposes. Also when the Group III nitride semiconductor is used in an electronic device, it has the potential to achieve superior properties compared to a case where a conventional Group III compound semiconductor is used.
Such a Group III nitride semiconductor is commonly produced by a metal-organic chemical vapor deposition (MOCVD) method using materials such as trimethyl gallium, trimethyl aluminum and ammonia. The MOCVD method is a method in which vapors of materials are contained in a carrier gas and are transported to the substrate surface, and then the materials are decomposed are decomposed by the reaction with the heated substrate, thereby causing crystal growth.
Heretofore, single crystal wafers of the Group III nitride semiconductor have not been commercially available, and the Group III nitride semiconductor is commonly obtained by growing crystals on a single crystal wafer of a different material.
It is also studied to manufacture Group III nitride semiconductor crystals by sputtering. For example, there is proposed a method in which a film of GaN is directly formed on a sapphire (Al2O3) substrate using a sputtering method for the purpose of laminating high-resistance GaN (for example, refer to Patent Document 1). When the film of GaN is formed by the sputtering method, there are advantages such as low equipment costs and stabilization of steps.
There is also proposed a method in which a GaN layer is formed on the (100) plane of silicon (Si) and the (0001) plane of sapphire (Al2O3) by high frequency magnetron sputtering using an N2 gas (for example, refer to Non-Patent Document 1).
There is also proposed a method in which a GaN layer is formed using an apparatus including a cathode and a solid target disposed to face with each other, a mesh being interposed between the substrate and the target (for example, refer to Non-Patent Document 2).
When a layer composed of crystals of the Group III nitride compound semiconductor described above is formed, there is required a technique of adding a dopant element such as Si or Mg for the purpose of controlling electric resistance. For example, there is an example of a report about the results of a test of doping in a GaN film using a sputtering method (for example, refer to Non-Patent Document 3). Non-Patent Document 3 does not describe a specific doping method and therefore it is impossible to perform an additional test by third party.
[Patent Document 1]    Japanese Unexamined Patent Application, First Publication No. S60-039819
[Non-Patent Document 1]    Y. USHIKU et al., “21st Seiki Rengo Symposium Ronbunshyu”, Vol. 2nd, p. 295 (2003)
[Non-Patent Document 2]    T. Kikuma at al., “Vacuum”, Vol. 66, p. 233 (2002)
[Non-Patent Document 3]    Edited by The Japan Society of Applied Physics, Pamphlet of “The 66th autumn meeting of the Japan Society of Applied Physics”, 7a-N-6, p. 248, 2005